In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, when a network device acting as a wireless device, such as a user equipment, is powered on, or when it moves between cells during a handover procedure, it receives and synchronizes to downlink signals as transmitted by a network device acting as a radio access network node, such as an evolved node B, in a cell search procedure. One purpose of such a cell search procedure is to identify the best cell or find the target cell for the wireless device and to achieve time and frequency synchronization to the network, as represented by the radio access network node, in the downlink (i.e. from the radio access network node to wireless device).
Primary and Secondary Synchronization Signals (PSS and SSS, respectively) are examples of signals used during cell search. FIG. 9 schematically illustrates transmission of PSS 101 and SSS 102.
A simplified initial cell search and handover procedure is illustrated in FIG. 10. Description thereof will now follow. A wireless device has typically a frequency error of 2 to 20 ppm (Part Per Million) at power on, which corresponds to 4 to 40 kHz frequency error at a carrier frequency of 2 GHz. The wireless device then tries to detect PSS from which it can derive the cell ID within a cell-identity group, which consists of three different cell identities corresponding to three different PSS. In this detection, the wireless device thus has to blindly search for all of these three possible cell identity groups. The wireless device also achieves an orthogonal frequency-division multiplexing (OFDM) symbol synchronization and a coarse frequency offset estimation with an accuracy of about 1 kHz. The latter is estimated by the wireless device by evaluating several hypotheses of the frequency error.
The wireless device can then continue to detect the SSS from which it acquires the physical cell ID and achieves radio frame synchronization. Here, the wireless device also detects if normal or extended cyclic prefix is used. If the wireless device is not preconfigured for either time-division duplexing (TDD) or frequency-division duplexing (FDD), the wireless device can detect the duplex mode by the position in the frame of detected SSS in relation to detected PSS. Fine frequency offset estimation can be estimated by correlating PSS and SSS. Alternatively, this fine frequency offset estimation is estimated by using the Cell specific Reference Signals (CRS).
After these synchronizations, the wireless device can receive and decode the Master Information Block (MIB) which is transmitted on the Physical Broadcast Channel (PBCH). Additional broadcasted information is transmitted in the System-Information Blocks (SIBs) which are carried by Physical Downlink Shared Channel (PDSCH). This PDSCH can be decoded after reading the Physical Control Format Indicator Channel (PCFICH) and the Physical Downlink Control Channel (PDCCH). Here, SIB2 includes information regarding uplink cell bandwidth and random access configurations. Thus, after successful decoding of SIB2, the wireless device can transmit a preamble on the Physical Random-Access Channel (PRACH) to the radio access network node and receive a Random Access Response (RAR) on the PDSCH from the radio access network node.
In general terms, PRACH is used for initial access and timing offset estimation for a wireless device. Upon reception in the radio access network node, the PRACH must thus be detected with high accuracy and an accurate timing offset estimation must be done. One PRACH can allocate several sub-frames, which is beneficial in large cells in order to improve coverage. However, one common (and smallest) allocation is to use one sub-frame. Here the PRACH is also configured with a periodicity, from once every second frame to once every subframe, i.e. with an interval spanning from 20 ms to 1 ms.
One mechanism to improve coverage of cell search signals is to use several antenna elements such that beamforming can be used to improve the signal-to-interference-plus-noise ratio (SINR), see illustration in FIG. 11. FIG. 11 schematically illustrates transmission of SSS, PSS, MIB, SIB and PRACH. A directional cell search procedure is proposed by C. Nicolas Barati, S. Amir Hosseini, Sundeep Rangan, Pei Liu, Thanasis Korakis, Shivendra S. Panwar in “Directional Cell Search for Millimeter Wave Cellular Systems”, Cornell University Library. Here the radio access network node periodically transmits synchronization signals in random directions to scan the angular space.
The need for designing synchronization and broadcast signals used in the initial cell search for scanning over a range of angles is also discussed by Sundeep Rangan, Theodore S. Rappaport, and Elza Erkip in “Millimeter-Wave Cellular Wireless Networks: Potentials and Challenges”, in the Proceedings of the IEEE, Volume: 102, Issue: 3, 2014, Page(s): 366-385.
However, the average time might be comparatively large until a wireless device receives a broadcast signal with high enough signal-to-noise ratio (SNR) with random or sequential beamforming of the broadcast signals and channels. Furthermore, when many wireless devices simultaneously receive the broadcast signals and channels, these wireless devices might also try to access the radio access network node by the PRACH channel simultaneously. This may result in congestion.
The inventors of the herein disclosed embodiments have realized that at least the issues disclosed above may be mitigated, or even resolved, by the use of adaptive beamforming. Hence, there is a need for an improved adaptive beamforming.